Much research has been done to develop catalysts that reduce the amounts of CO, hydrocarbons and NO.sub.x emitted to the atmosphere in the exhaust gases of internal combustion engines. More recently, because automobile manufacturers have chosen to produce "lean-burn" engines to improve fuel economy, this research has concentrated on reducing those emissions from internal combustion engines that operate lean. Catalysts that were found to be effective in reducing emissions of CO, hydrocarbons (HC), particulate matter (PM) and NO.sub.x (i.e., "Four-Way Catalysts") emitted from engines operating with a stoichiometric amount of combustion air have, in many cases, been found to be less effective in reducing NO.sub.x in exhaust gases from lean-burn engines, particularly after the catalysts have been used for a period of time, i.e., aged.
A good deal of the most recent research in emissions control catalysts has been focused on the control of hydrocarbon emissions during the period shortly after engine start-up, often referred to as "cold start". For example, U.S. Pat. No. 5,284,638 (Hertl, et al.), U.S. Pat. No. 5,662,869 (Abe, et al.) and U.S. Pat. No. 5,814,287 (Adamczyk, Jr., et al.) teach the use of a hydrocarbon adsorbent along with a conventional three-way catalyst, while U.S. Pat. No. 5,772,972 (Hepburn, et al.) teaches a hydrocarbon adsorbent along with a Pd-based three-way catalyst.
Other researchers of lean-burn catalysts have focused their efforts on various metal systems on conventional supports. For example, U.S. Pat. No. 5,618,505 (Subramanian, et al.) teaches a two-stage catalyst system comprising a first stage of tungsten on .gamma.-alumina and a second stage of .gamma.-alumina with Cu, Co, Fe, or Mn dispersed thereon.
Many patents have been granted on lean-burn catalysts of various metals on zeolite supports also. For example, U.S. Pat. No. 5,270,024 (Kasahara, et al.) teaches a catalyst comprising a zeolite with a silica/alumina ratio of at least 10 with Cu and one rare earth dispersed thereon; U.S. Pat. No. 5,407,651 (Kawai) teaches a catalyst comprising a zeolite, and optionally alumina, having Mn, Co+Pd, or Co+Ag dispersed thereon; U.S. Pat. No. 5,443,803 (Mizuno, et al.) teaches a two-stage catalyst comprising zeolite with Co and an alkaline earth metal and zeolite with Cu; U.S. Pat. No. 5,409,671 (Takemoto, et al.) teaches a two-layer catalyst, the first layer being a Na-type zeolite having a silica/alumina ratio of 30 impregnated with Cu and a second layer of zeolite impregnated with Cu and Zr; and U.S. Pat. No. 5,433,933 (Eshita, et al.) teaches a zeolite having a silica/alumina ratio of at least 15 impregnated with Co and optionally an alkaline earth metal or rare earth metal.
None of these researchers have investigated the character of the zeolite used in their catalysts beyond those few who have specified a minimum silica/alumina ratio. This invention teaches that the performance, both new and when aged, of lean-burn catalysts is improved when particular zeolites are used in the support formulation.